Changes rarely have single effects. One little thing, such as the way we count levels, can ripple through a game causing changes far beyond the level cap.

Previously I complained that the expansions had a leveling cutoff: once you reached the cap for that expansion, xp dropped by 90% for the mobs in the area. It seemed pointless to just cut out content. Besides, there was already the gear issue: new expansions give much better gear, so there is already a carrot to bring people to the new content, making a stick seemingly redundant. The gearing was also a stick, since the Cataclysm instances all have ilevel restrictions so that to get in you need to run the new zones anyway.

The problem, as was pointed out by a commenter, was that without the xp depreciation it would be faster to level to 85 using Northrend mobs than the newer and tougher Cataclysm enemies. I'll leave aside "so what?" as a counter-argument and just run with the assumption that speed-leveling to 85 must happen in the Cataclysm zones. Here is where we get to one of the Five Level Problems. With a wider range of levels we'd naturally see a drop-off in the experience from Northrend mobs, which would eventually become grey and not long before that, green. This would leave a short distance where perhaps Northrend mobs are better, but not far, and attempting to jump ahead with them would leave the player undergeared from lack of quests and instances, slowing them before long (though a player with a great deal of ICC gear would be near the level
of Cataclysm gear, so maybe that's not true, at least not for a long
time). At the least, adding ten levels rather than five would reduce the relative speed of Northrend racing (and give more time to switch out ICC gear).

As we level up and get better gear, mobs level up and get stronger, so once we're in an expansion's gear, until we start running heroics, we're going to stay somewhat on par with mobs. They get stronger as fast as we do. With five levels that means we get stronger faster, and so do they. The result is that small differences in level are more significant. With ten levels, the small differences are diluted, so if you're off by a few levels you're not going to get rolled over, just suffer a worse attack table. For players this means more choice. Sick of Borean Tundra and Howling Fjord? Jump ahead to Dragonblight and it's not the end of the world. Back in Burning Crusade some players would skip straight to Zangarmarsh as a way to bypass the crowded Hellfire Peninsula. For leveling in Azeroth this gives a little more flexibility. In all cases, the level requirements to pick up quests don't help, locking out zone quests which are still possible for the player. The quest locking, extended to entire zones, was made worse with only five levels. There is the second Five Level Problem: if a level is even more significant, then whatever justification is used for the level requirement becomes stronger, so have more gates on content then we would otherwise.

Before I wrap this up, I want to leave a comment for commenters: I know some of you don't like gating or gearing or leveling. Those are all internally-generated opinions which are not caused by the five or ten levels added by an expansion. So please, don't comment about how gates are stupid and why can't we all just do the content we want to the instant we get the thought.

I don't know why Blizzard only added five levels. I'm sure Google could give me some statements from them, but I'm not sure that would actually explain anything. The fewer levels are consistent with the ever-stronger push to the level cap and away from leveling. This push is much more insistent, as rather than being an heirloom that you can avoid or excessive quest xp that you can grey out and ignore, having only five levels shoves players right into the level cap. But, maybe it's not so bad, since after all, so far all these "five level problems" are all leveling problems, not part of the "real game."